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.\" Copyright (c) 2011 Jonathan Anderson
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.Dd June 17, 2025
.Dt CAPSICUM 4
.Os
.Sh NAME
.Nm Capsicum
.Nd lightweight OS capability and sandbox framework
.Sh SYNOPSIS
.Cd "options CAPABILITY_MODE"
.Cd "options CAPABILITIES"
.Sh DESCRIPTION
.Nm
is a lightweight OS capability and sandbox framework implementing a hybrid
capability system model.
.Nm
is designed to blend capabilities with UNIX.
This approach achieves many of the beneﬁts of least-privilege operation, while
preserving existing UNIX APIs and performance, and presents application authors
with an adoption path for capability-oriented design.
.Pp
Capabilities are unforgeable tokens of authority that can be delegated and must
be presented to perform an action.
.Nm
makes file descriptors into capabilities.
.Pp
.Nm
can be used for application and library compartmentalisation, the
decomposition of larger bodies of software into isolated (sandboxed)
components in order to implement security policies and limit the impact of
software vulnerabilities.
.Pp
.Nm
provides two core kernel primitives:
.Bl -tag -width indent
.It capability mode
A process mode, entered by invoking
.Xr cap_enter 2 ,
in which access to global OS namespaces (such as the file system and PID
namespaces) is restricted; only explicitly delegated rights, referenced by
memory mappings or file descriptors, may be used.
Once set, the flag is inherited by future children processes, and may not be
cleared.
.Pp
Access to system calls in capability mode is restricted: some system calls
requiring global namespace access are unavailable, while others are
constrained.
For instance,
.Xr sysctl 2
can be used to query process-local information such as address space layout,
but also to monitor a system’s network connections.
.Xr sysctl 2
is constrained by explicitly marking \(~~60 of over 15000 parameters as permitted
in capability mode; all others are denied.
.Pp
The system calls which require constraints are
.Xr sysctl 2 ,
.Xr shm_open 2
.Pq which is permitted to create anonymous memory objects but not named ones
and the
.Xr openat 2
family of system calls.
The
.Xr openat 2
calls already accept a file descriptor argument as the directory to perform the
.Xr open 2 ,
.Xr rename 2 ,
etc. relative to; in capability mode the
.Xr openat 2
family of system calls are constrained so that they can only operate on
objects “under” the provided file descriptor.
.It capabilities
Limit operations that can be called on file descriptors.
For example, a file descriptor returned by
.Xr open 2
may be refined using
.Xr cap_rights_limit 2
so that only
.Xr read 2
and
.Xr write 2
can be called, but not
.Xr fchmod 2 .
The complete list of the capability rights can be found in the
.Xr rights 4
manual page.
.El
.Pp
In some cases,
.Nm
requires use of alternatives to traditional POSIX APIs in order to name
objects using capabilities rather than global namespaces:
.Bl -tag -width indent
.It process descriptors
File descriptors representing processes, allowing parent processes to manage
child processes without requiring access to the PID namespace; described in
greater detail in
.Xr procdesc 4 .
.It anonymous shared memory
An extension to the POSIX shared memory API to support anonymous swap objects
associated with file descriptors; described in greater detail in
.Xr shm_open 2 .
.El
.Pp
In some cases,
.Nm
limits the valid values of some parameters to traditional APIs in order to
restrict access to global namespaces:
.Bl -tag -width indent
.It process IDs
Processes can only act upon their own process ID with syscalls such as
.Xr cpuset_setaffinity 2 .
.El
.Pp
.Fx
provides some additional functionality to support application sandboxing that
is not part of
.Nm
itself:
.Bl -tag -width indent
.It Xr capsicum_helpers 3
A set of a inline functions which simplify modifying programs to use
.Nm .
.It Xr libcasper 3
A library that provides services for sandboxed applications, such as operating
on files specified on a command line or establishing network connections.
.El
.Sh SEE ALSO
.Xr cap_enter 2 ,
.Xr cap_fcntls_limit 2 ,
.Xr cap_getmode 2 ,
.Xr cap_ioctls_limit 2 ,
.Xr cap_rights_limit 2 ,
.Xr fchmod 2 ,
.Xr open 2 ,
.Xr pdfork 2 ,
.Xr pdgetpid 2 ,
.Xr pdkill 2 ,
.Xr pdwait4 2 ,
.Xr read 2 ,
.Xr shm_open 2 ,
.Xr write 2 ,
.Xr cap_rights_get 3 ,
.Xr capsicum_helpers 3 ,
.Xr libcasper 3 ,
.Xr procdesc 4
.Sh HISTORY
.Nm
first appeared in
.Fx 9.0 ,
and was developed at the University of Cambridge.
.Sh AUTHORS
.Nm
was developed by
.An -nosplit
.An Robert Watson Aq Mt rwatson@FreeBSD.org
and
.An Jonathan Anderson Aq Mt jonathan@FreeBSD.org
at the University of Cambridge, and
.An Ben Laurie Aq Mt benl@FreeBSD.org
and
.An Kris Kennaway Aq Mt kris@FreeBSD.org
at Google, Inc., and
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net .
Portions of this manual page are drawn from
.Rs
.%A Robert N. M. Watson
.%A Jonathan Anderson
.%A Ben Laurie
.%A Kris Kennaway
.%T Capsicum: practical capabilities for UNIX
.%J USENIX Security Symposium
.%D August 2010
.%O DOI: 10.5555/1929820.1929824
.Re
